1. Field of the Invention
The present invention relates to a motor drive control apparatus configured to drive and control a motor with a supply of electric power from a direct-current power source, as well as a vehicle equipped with such a motor drive control apparatus and a corresponding motor drive control method.
2. Description of the Related Art
One known structure of a motor drive device includes a booster converter equipped with a reactor and a switching element to boost a direct-current voltage supplied from a direct-current power source, an inverter configured to convert an output voltage of the booster converter into an alternating-current voltage and drive an alternating-current motor with the converted alternating-current voltage, and a controller configured to control the booster converter and the inverter (see, for example, Japanese Patent Laid-Open No. 2007-166875 and No. 2006-311768). The motor drive device of this known structure controls the inverter by selectively applying one of sine-wave PWM control of setting a modulation factor of voltage conversion by the inverter to a range of 0 to 0.61, overmodulation PWM control of setting the modulation factor to a range of 0.61 to 0.78, and rectangular-wave control of setting the modulation factor to a fixed value of 0.78. The motor drive device refers to the modulation factors of the inverter in the respective controls and adopts the sine-wave PWM control in a low rotation speed zone of the operation range of the alternating-current motor and the overmodulation PWM control in a middle rotation speed zone, and the rectangular-wave control in a high rotation speed zone. Such selective application of the control mode reduces a torque variation and allows the smooth output characteristic in the low rotation speed zone of the alternating-current motor, while enhancing the output of the alternating-current motor in the middle and high rotation speed zones.
The rectangular-wave control adopted in the motor drive device of the prior art structure has the lower control accuracy (the poorer control response) than the sine-wave PWM control. The rectangular-wave control, however, enhances the output of the alternating-current motor and improves the energy efficiency with prevention of a copper loss and a switching loss. Expansion of the application range of the rectangular-wave control in the operation range of the alternating-current motor is expected to improve the performance of the electric drive system including the alternating current motor and the energy efficiency. In the motor drive device of the prior art structure including the booster converter, smoothing capacitors are provided on both the direct-current power source side and the inverter side of the booster converter. The reactor of the booster converter and these smoothing capacitors constitute a resonance circuit. When the operation point of the alternating-current motor is included in a specific area, resonance of voltage or current occurs in the booster converter. In the expansion of the application range of the rectangular-wave control, the potential resonance of voltage or current occurring in the booster converter should be considered, in order to prevent overvoltage or overcurrent from being applied to or flowed through the booster converter and the smoothing capacitors.